1. Field of the Invention
The present invention is directed to monitoring power of a light emitting device, specifically for a diffractive power monitor for use with vertical cavity surface emitting lasers (VCSELs).
2. Description of Related Art
Light emitting devices such as VCSELs need some form of power control to maintain a constant output. Such control is typically automatically performed by measuring an output of a light emitting device during operation and using this measurement to control the power supplied to the light emitting device.
Such control may be easily achieved when the light emitting device to be controlled is an edge emitting laser because edge emitting lasers output light from two ends thereof. Thus, one output may be used for the desired application, while the other output may be used for the power control.
In contrast, a VCSEL typically only emits light from one surface. Hence, any monitoring of the light must be from the same output as used for the desired application of the VCSEL. VCSELs are much cheaper and their surface emissions make them easier to integrate with other optical devices than the edge emitting lasers, so the use of VCSELs is very desirable.
Previous attempts to monitor the power of VCSELS typically involve splitting off of a portion of the output beam to use as a monitor beam. Examples of such configurations are disclosed in U.S. Pat. Nos. 5,757,836 and 5,774,486. However, such splitting off obscures part of the beam which may affect the wavefront and imaging, and hence coupling, of the light. Further, if the intensity distribution changes, such as when there is a change in lasing mode, the monitored power may change in a way which does not represent the overall output power of the VCSEL.
Additionally, splitting off of the beam may require the output of the VCSEL to be increased in order to maintain the requisite power level while allowing detection. Previous uses of scattering the beam to create a monitor beam relied on reflection for directing the beam and did not provide an optimal signal to the monitor detector. Further, previous scattering did not insure the entire beam was being monitored.